The objective of this study is to provide a firm biochemical basis for understanding the mechanism involved in hemostasis and thrombosis. The binding of thrombin to human platelets and the manner in which the stimulus may be coupled to the release reaction are under investigation. In addition, the properties and origin of platelet fibrinogen and the role of contractile proteins in platelet function will be studied. Some of these studies with normal platelets are then extended to platelets with defective functions.